[kcms/virtualdesktops] Properly track synchronizing state
When the user applies a change like adding a desktop DesktopsModel::syncWithServer is called and m_synchronizing is set to true. The server then sends some kind of response (e.g. desktopCreated) and updateModifiedState is called which sets m_synchronizing to false again.
However if a setting is changed that does not trigger any server change (e.g. the show OSD setting) m_synchonizing is never set to false again, causing problems down the line.
Instead of relying on updateModifiedState to reset m_synchronizing set it back when the last DBus call finished or at the end of syncWithServer if no call was made.